This invention pertains to treatments for reducing the salt passage through reverse osmosis membranes.
Removal of materials dissolved or dispersed in a liquid medium by separation of these materials from the dispersing medium or solvent utilizing reverse osmosis processes is well known. Reverse osmosis processes utilize membranes which are selectively permeable to certain components of the solution or mixture. A chemical potential is established across the membrane such that components to which the membrane is permeable will diffuse from a first surface through the membrane to a space in contact with the opposite side of the membrane. Conventionally, aqueous solutions of sodium chloride or other salts are treated by a reverse osmosis process to produce a water permeate of reduced salt content.
A wide variety of materials have been disclosed in the prior art to be useful as reverse osmosis membranes. Two of the more widely used reverse osmosis membrane materials are cellulose esters and polyamide membranes. U.S. Pat. Nos. 3,494,780 and 3,532,527 describe processes for making cellulose ester membrane useful in reverse osmosis processes. Cellulose acetate membranes have enjoyed considerable commercial success.
Polyamide membranes have also been employed commercially in reverse osmosis processes. U.S. Pat. No. 4,277,344 describes a variety of membranes having polyamide discriminating layers. Thin film composite membranes having crosslinked polyamide discriminating layers have proven particularly useful in reverse osmosis processes. Such membranes are further illustrated in U.S. Pat. Nos. 4,520,044 and 4,606,943. Other polyamide membranes of interest are described in U.S. Pat. Nos. 3,551,331 and 3,567,632.
A variety of other materials have been evaluated in the prior art as discriminating layers for reverse osmosis membranes. For example, sulfonated polysulfone, crosslinked furfuryl alcohol derivatives, crosslinked polyvinyl alcohol and crosslinked alkyl/aryl polyureas can be used as reverse osmosis membranes. Other reverse osmosis membranes are described in U.S. Pat. Nos. 3,744,642; 3,926,798; 4,005,012; 4,366,062 and 4,634,531.
Techniques for manufacturing reverse osmosis membranes frequently produce membranes having a plurality of defects or discontinuities in the discriminating layer. Conventionally, commercial reverse osmosis membranes have been treated to plug or seal these defects. U.S. Pat. No. 3,877,978 discloses the use of copolymers of vinyl acetate with various ethylenically unsaturated carboxylic acids to enhance the rejection of semi-permeable membranes. U.S. Pat. No. 3,886,066 describes the use of hydrolyzable tannins to reduce the salt passage through semi-permeable membranes. U.S. Pat. No. 4,214,020 describes a process for coating the exteriors of hollow fiber membranes ot enahnce their membrane characteristics. U.S. Pat. No. 4,634,531 describes the use of sequential treatment with a water-soluble amine and a water-soluble aldehyde to improve selective permeation. The prior art techniques can be used to significantly reduce salt passage through reverse osmosis membranes but generally result in a significant loss of water flux through the membrane.
A report available from NTIS designated PB81-170607, published Jan. 31, 1981, describes a variety of agents for regenerating reverse osmosis membranes. Polyacrylamide was evaluated, but in regeneration of a polyamide membrane commercially available from DuPont was found ineffective as noted on page 42. Polyacrylic acid and cationic, anionic and nonionic polyacrylamide did increase the rejection of deteriorated poly(ether/amide) membranes as apparent from Table 5-1 on page 42. Cationic polyacrylamide also increased salt rejection of new cellulose diacetate membranes, as apparent from page 53.
Another report available from NTIS designated PB82-157975, submitted in January 1981, discloses post-treatment processes for polyamide membranes. A variety of reagents including polyacrylamide (see page 8) were used with perforated composite polyamide membranes. These polyamide membranes had a desalinizing layer prepared by reaction of trimesoyl chloride and m-phenylenediamine. The polyacrylamide treatment did not restore the desired rejection of greater than 90 percent.
Japanese Kokai 55-011,048 describes vinyl polymers bearing ether, carboxylic acid and carboxylic acid ester groups which enhance rejection of cellulose acetate reverse osmosis membranes without lowering water flux.
The art has long sought techniques to reduce salt passage through a reverse osmosis membrane without a significant loss of water flux through the membrane. A technique for restoring or enhancing performance of reverse osmosis membranes immediately after manufacture would make the operation of such membranes more economical and reduce the production of products which do not meet the requisite membrane performance specifications.